1. Field of the Invention
This invention generally relates to nonlinear optics applications. More particularly, this invention relates to ovens for maintaining crystals at a desired operating temperature for optics applications.
2. Description of the Related Art
There are many nonlinear optics applications which require heating a crystal to a specific operating temperature and maintaining the heated crystal at that specific operating temperature or at a programmed sequence of predetermined temperatures over a long period of time. Frequency doubling arrangements, for example, require critical temperature control. This is discussed in U.S. Pat. No. 4,019,159, "Optical Frequency Doubler Using Electro-Optic Crystal with Improved Feedback Control of Crystal Phase Match", issued Apr. 19, 1977, and assigned to Hughes Aircraft Company, the assignee of the present invention.
Optical frequency doubling can be achieved by passing a laser beam through a crystal of a material which develops laser beam components at harmonic frequencies of the frequency of the input beam. Maximum conversion of energy to the double frequency (second harmonic) beam occurs when light propagates through the crystal with a phase velocity at the fundamental frequency equal to the phase velocity at the second harmonic frequency. This phase-match condition is achieved when the crystal has an index of refraction for light at the fundamental frequency equal to the index of refraction for light at the second harmonic frequency.
Two key parameters affect the indices of refraction of these crystals and hence the frequency doubling efficiency: firstly, the physical alignment of the crystal with respect to the incoming radiation and secondly, the temperature of the crystal. A slight change in either one of these parameters can reduce the amplitude of the doubled frequency output considerably. Maintaining the crystal in the proper alignment is relatively straightforward as long as the alignment tolerance angle is not too small. However, maintaining the temperature of the crystal at the critical value for which the desired phase match occurs has been problematic.
One of the most widely used nonlinear crystals for frequency doubling applications is Potassium Dihydrogen Phosphate (KD*P), however, the nonlinear crystal Cesium Dideuterium Arsenate (CD*A) is actually more desirable for frequency doubling applications. CD*A has the property of 90.degree. phase matching for doubling of 1.06 .mu.m lasers. The angular tolerance for the beam divergence for CD*A is about 50 times larger than that for KD*P, and therefore, CD*A accepts beams with relatively larger angular spread than KD*P. CD*A does not alter the beam polarization, and can be readily used in phase conjugated oscillator/amplifiers. CD*A has a much higher damage threshold than Lithium Niobate and Barium Sodium Niobate which also allow 90.degree. phase matching for 1.06 .mu.m lasers. Therefore, CD*A is more attractive for medium energy and power frequency doubling applications than the more widely used KD*P. In spite of the potential advantages and superior performance of CD*A, however, CD*A crystals have not been widely used because of numerous problems associated therewith.
The phase match temperature for CD*A is approximately 110.degree. C. In the past, when CD*A was operated at 110.degree. C. for extended periods of time, degradation of the crystal surfaces occurred, eventually resulting in device breakdown. Additionally, the phase match temperature and deuteration level of CD*A were found to fluctuate considerably.
Ovens have been employed to heat the crystal to the desired phase-match temperature. As the laser beam traverses a crystal, however, sufficient absorption-induced heating of the crystal occurs to destroy the phase-match condition that has been created. Although the oven temperature can be controlled in response to a measured temperature, the thermal time constants inherent in direct oven temperature control servos are too slow to be practical. Therefore, a common practice in the industry has been to preset the oven temperature a few degrees below the desired phase-match temperature and allow the crystal heating due to interaction with the laser beam to raise the crystal temperature to substantially the phase-match temperature. Such preset oven temperature arrangements, however, require substantial start-up times before the crystal reaches the desired operating temperature, and in addition, they do not provide continuous control over the double frequency output amplitude. Therefore, although this approach has attempted to solve the problem of absorption-induced heating, it has not addressed the other problems of using CD*A crystal.
One approach to compensating for the deleterious optical effects of crystal self-heating in nonlinear crystals, such as cesium dideuterium arsenate (CD*A), is disclosed in U.S. Pat. No. 4,181,899, "High Power Optical Second Harmonic Generation in NonLinear Crystals", inventor Y. S. Liu, issued Jan. 1, 1980. The Liu approach involves tuning the laser output frequency in accordance with sensed crystal temperature, to compensate for the phase mismatch caused by crystal self-heating when the laser radiation is incident on the crystal. However, this approach is not satisfactory in cases where fixed output frequencies are required.
Another approach disclosed in U.S. Pat. No. 4,019,159, provides a feedback control arrangement responsive to both the magnitude of the double frequency component and to the temperature within the oven. The indices of refraction of light in the frequency doubling electro-optic crystal are controlled by both electric field pulses applied across the crystal and heater control pulses for controlling the temperature of the oven in which the crystal is mounted. This approach provides much faster temporal response than the previously discussed approach.
Although both of these approaches may be used in the oven of the present invention, neither approach by itself satisfactorily provides for rapid and controlled heat dissipation, and rapid sensing and control of the crystal temperature.
It has been previously noticed that heating the CD*A crystal in a sealed cell over a period of time result in the deposition of a thin liquid film on the inside surface of the cell windows which were cooler than the crystal. This condensation caused laser damage to the windows and eventually the crystal. One prior approach to solving this problem was to continuously purge the oven chamber with dry nitrogen to prevent condensation on the windows. This approach, however, actually accelerated crystal degradation. Another approach was to use an open cell so that the vapor could leave the cell. This approach, however, made it difficult to uniformly heat the crystal and maintain the entire heated crystal at the desired operating temperature over a period of time.
Highly deuterated CD*A has been recognized to be highly desirable for frequency doubling applications due to significant reduction in absorption. However, thermal degradation of the highly deuterated CD*A has continued to be a problem. See Y. S. Liu et al., "Specific Heat of Cesium Dideuterium Arsenate (CSD.sub.2 As0.sub.4) from 0.degree. to 120.degree. C.", Appl. Phys. Lett., Vol. 27, No. 11, pp. 585-587. Prior proposed solutions to this problem have had other undesirable side effects or drawbacks. For example, it has been suggested that a less highly deuterated material may be used. Such a material has a comparatively lower phase-match temperature, and therefore thermal degradation induced by heating a more highly deuterated crystal to a higher phase-match temperatures can be avoided. Unfortunately, however, less highly deuterated CD*A has more absorption, and this offsets the previously stated advantage.
Another approach is the use of highly deuterated, low absorption CD*A with angle phase matching. Highly deuterated CD*A operates noncritically phase matched at 110.degree. C. or higher, but can be angle tuned at lower temperatures, for example 80.degree.-90.degree., depending on the deuteration level. However, operation at lower temperatures offsets some of the advantages of CD*A over KD*P, such as wide acceptance angle and absence of walk-off.
The previous approaches to modifying the crystal-heating ovens to solve the above-discussed problems associated with the use of CD*A have not been entirely satisfactory. Therefore, there continues to be a need for an apparatus capable of uniformly heating a crystal or maintaining the heated crystal at a specific operating temperature for extended periods of time.